Typically, optical polarization state measurement methods are based on measurements of the individual Stokes vector components, i.e., measurements of the optical power transmitted through 0xc2x0 linear, 45xc2x0 linear and circular polarizers. The analogous technique has been proposed in the heterodyne architecture by I. Roudas et al. in xe2x80x9cCoherent heterodyne frequency-selective polarimeter for error signal generation in higher-order PMD compensators,xe2x80x9d OFC 2002, pp. 299-301, where the polarization state of the local oscillator is sequentially switched between 0xc2x0 linear, 45xc2x0 linear and circular polarization states to provide heterodyne measurements of the signal amplitude in the selectable polarization states. The polarization switching that is required slows down the polarization measurement process.
Another heterodyne technique determines the polarization state by determining the amplitude and relative phase of the two detected heterodyne signals. This technique has been used by K. Oka et al., xe2x80x9cEvaluation of phase fluctuations of orthogonal optical eigen modes guided in an axially vibrating birefringent single-mode fiberxe2x80x9d, Journal of Lightwave Technology, Vol. 8, No. 10, 1482-1486, 1990 to determine fiber birefringence and by C. Chou et al., xe2x80x9cAmplitude sensitive optical heterodyne and phase lock-in technique on small optical rotation angle detection of chiral liquidxe2x80x9d, Japanese Journal of Applied Physics, Part 1, Vol. 36, No. 1A, 356-359, 1997 to measure optical activity in chiral liquids at fixed optical frequencies. However, this heterodyne technique has not been used with swept local oscillator sources.
An optical heterodyne system is inherently sensitive to the polarization of the heterodyned signals. In accordance with the invention, a polarization state is measured by using a polarization diversity receiver employing a polarization beam splitter to output two heterodyne signals. The amplitude and relative phase of the two detected heterodyne signals uniquely determine the polarization state. However, a problem arises when the local oscillator (LO) is swept over a frequency range and not kept at a fixed frequency. The polarization state is no longer uniquely determined but jumps between the hemispheres of the Poincare sphere creating a polarization state ambiguity.
Modification of the detection method and apparatus in accordance with the invention eliminates the polarization state ambiguity arising from the two images that result from the mixing process thus allowing unambiguous determination of the polarization state.